1. Field of the Invention
The present invention relates to a control method and apparatus for a continuously variable transmission. More specifically, the present invention relates to a control method and apparatus adapted to be used with a belt-type continuously variable transmission.
2. Description of the Background Art
Many types of transmissions are known for transmitting power from an engine to a driving wheel of a vehicle. One example of a known belt-type continuously variable transmission, for transmitting an engine driving force to a driving wheel, includes a centrifugal starting clutch provided on an output shaft (a driven side). In such an arrangement, an input of torque from the driving wheel to the transmission can be temporarily interrupted when the vehicle is brought to a sudden stop. Accordingly, a gear ratio of the continuously variable transmission can be shifted to a low speed condition for starting (a low side) if the engine keeps on running. A torque reduction is, however, applied to the output shaft of the continuously variable transmission at this time. This results in a need for increasing the capacity of the transmission's starting clutch, leading to an increase in the dimensions or weight of the mechanism.
In another example of a known transmission, a starting clutch is provided on an input shaft (a driving side) of the continuously variable transmission. In such an arrangement, the gear ratio of the continuously variable transmission does not return to the low speed side, even when the vehicle is brought to a sudden stop. In such a case, a new technical problem arises. Specifically, sufficient acceleration performance cannot be obtained, since the gear ratio of the continuously variable transmission is not in the low speed side when the vehicle is restarted.
Japanese Laid-open Patent No. 2003-14004 discloses a transmission system that accomplishes the following operation. Specifically, a selector mechanism for connecting and disconnecting the transmission of power to a driving wheel is provided on an output shaft of a continuously variable transmission. The selector mechanism is controlled so as to temporarily disconnect the transmission power from the wheels, when a vehicle is brought to a sudden stop. Prior to this disconnection of transmission power, a starting clutch is released. The power from the continuously variable transmission to the driving wheel is thereafter disconnected. After the disconnection of transmission power, the starting clutch is engaged again.
The prior art apparatus described above prevents the engine from stalling, when the vehicle is brought to a sudden stop. There is, however, a technical problem that needs to be addressed. Specifically, if the engine stops before the gear ratio of the continuously variable transmission is shifted to the low speed side, the vehicle has to be started next time with the transmission at a position other than the low speed position. As a result, a good acceleration performance cannot be obtained.
It is therefore an object of the present invention to solve the technical problem of the prior art, and to provide a control method and apparatus for a continuously variable transmission ensuring good acceleration performance during a starting operation which follows an engine stall, occurring before a gear ratio of the continuously variable transmission is shifted back to a low speed side.